Circuit-breaker with pressure balanced relatively movable contacts

ABSTRACT

A circuit-breaker is provided with relatively movable contacts and an actuating rod is attached to one of the contacts to effect separation of the contacts and opening of the circuit-breaker. The contact assembly is housed within an outer chamber which is subjected to insulating gas under high pressure and the contact assembly forms an inner chamber which, when the contacts are closed, is isolated from the outer chamber and which is vented to a pressure lower than the pressure in the outer chamber. A small net force acts on the movable contact to which the actuating rod is attached to maintain the contacts in closed condition. When the contacts are closed, the pressure differential existing between the outer and inner chambers is used in substantially balancing fashion and additional means is provided for cooperating with this differential pressure effect to establish the small net force serving to maintain the contacts in closed condition. When the contacts are open the pressure differential disappears and full pressure balance is effected so that both upon initial opening and continued opening movement as effected by the actuating rod, the actuating rod is subjected to very little mechanical stress. The opening of the contacts opens a valve formed between separable contacts and communicating the inner chamber with the outer chamber and allowing the insulating or extinguishing gas to rush past the contacts and axially through at least one of them for extinguishing the arc produced by opening of the contacts.

I United States Patent inventors Gijsbert W. Irik CIRCUIT BREAKER WITH PRESSURE BALANCED RELATIVELY MOVABLE CONTACTS 9 Claims, 3 Drawing Figs.

US. 200/148 Int. Cl H0lh 33/80 Field of Search 200/148 References Cited FOREIGN PATENTS 638,785 10/1963 Belgium 200/148 1,055,644 4/1959 Germany ZOO/148.2

Primary Examiner-Robert S. Macon I Attorneylmirie and Smiley ABSTRACT: A circuit-breaker is provided with relatively movable contacts and an actuating rod is attached to one of the contacts to effect separation of the contacts and opening of the circuit-breaker. The contact assembly is housed within an outer chamber which is subjected to insulating gas under high pressure and the contact assembly forms an inner chamber which, when the contacts are closed, is isolated from the outer chamber and which is vented to a pressure lower than the pressure in the outer chamber. A small net force acts on the movable contact to which the actuating rod is attached to maintain the contacts in closed condition. When the contacts are closed, the pressure differential existing between the outer and inner chambers is used in substantially balancing fashion and additional means is provided for cooperating with this differential pressure effect to establish the small net force serving to maintain the contacts in closed condition. When the contacts are open the pressure differential disappears and full pressure balance is effected so that both upon initial opening and continued opening movement as effected by the actuating rod, the actuating rod is subjected to very little mechanical stress. The opening of the contacts opens a valve formed between separable contacts and communicating the inner chamber with the outer chamber and allowing the insulating or extinguishing gas to rush past the contacts and axially through at least one of them for extinguishing the are produced by opening of the contacts.

FIGJI INVENTORS v. rm

ATTORNEYS sum 1 {IF-12 DDK.

m lit N g l v I k :E 1

Mm mwwu mm f "a nu mm nwmw mm mm aw mm MW .PATENTED FEB] 6 I971 RIINTJ'E BoERsMA GIJSBERT BY p PATENTED FE816I97| sum 2 OF 2 u. 0 ooll g FIGS F'IG.2

IllIlIl-ll.

INVENTORS} BDERSMA QINTJE I Gm's BERT w- IR The invention relates to a gas blast circuit-breaker comprising a switching chamber filled with extinguishing gas under pressure and containing at least one pair cooperating contacts which are carried by fixed contact holders, at least one of said contacts having a central passage for the discharge of extinguishing gas during the opening operation and at least one of said contacts being a movable contact and forming together with its contact holder a cylinder-piston-system which is loaded by the extinguishing gas and by means of which said movable contact is driven in the opening direction by the extinguishing gas during the opening operation, said movable contact forming the movable part of a valve-system formed by said movable contact and the other contact or the holder thereof for keeping said switching chamber and said discharge passage separated from one another, when the circuit-breaker is in its closed position, and said movable contact being loaded during its closing movement by a readjusting force.

Gas blast circuit-breakers of this kind are mostly adapted to interrupt the electric circuit and to extinguish the switching arc only. in the condition of rest their cooperating contacts are in contact with each other, so that such circuit-breakers can only be u'sedwhen connected in series with an isolatorswitch for keeping the circuitinterrupted and for closing same. These gas blast circuit-breakers have the advantage that the movable contact with all accessories is very light and has only to be moved over a small distance and that extinguishing gas of relatively high pressure is at once available to separate the contacts and to extinguish the switching arc. However, these circuit-breakers have also some disadvantages. For instance, the controlling of theisolator-switch connected in series therewith is not positively connected with the circumstances in the gas blast circuit-breaker, so that it can happen, that'when the circuit-breaker, after an opening command has been given, responds with hesitation to said command the isolator-switch opening independently of said fact the predetermined space of time after saidcommand is put in operation before the switching are set up in the circuitbreaker has been extinguished. Another difficulty is that the position of the movable contact cannot be observed from the outside. ln circuit-breakers for high tension provided with multiple interruption, consequently in circuit-breakers comprising a number of pairs of cooperating contacts connected in series, the disadvantage exists, that the movablejcontacts can move independently 'of one another, so that there is no certainty, that the interruption of the electric circuit will happen in all places of interruption at the same or substantially the same time.

The invention has for its object to provide a gas blast circuit-breaker of the'described kind which has the good points but not the mentioned disadvantages of the known circuitbreakers. It consists in that the cylinder-piston-system and the valve-system are so constructed and the readjusting force is so chosen as to produce in the closed conditionof the circuitbreaker a holding force to keep said movable contact and said valve-system in their closed positions and as to allow the extinguishing gas to drive the movable contact towards its open position immediately after the valve-system has been opened, said movable contact being connected by mechanical coupling means to anex'ternal auxiliary driving mechanism, by means of which said movable contact is moved, at least at the beginning of the opening operation, against said holding force and said valve-system is opened. In this circuit-breaker the movable contact is held in its closed position by a difference of forces and the auxiliary driving mechanism has only to exert a slightly stronger force than this difference of force. But, as soon as the valve-system has been opened, the component of the force exerted on the valve-system by the extinguishing gas and counteracting the opening movement ceases to exist, so

that the extinguishing gas then acts only in the opening direction on the movable contact and said contact is driven with great speed in said direction by the great difference of pressure'which, at the beginning of the opening operation, is set up over the piston cooperating with said contact. The readjusting force has only to be small in respect of the force exerted in the opening direction on the movable contact by the extinguishing gas and it may even be reduced to zero during the opening operation. Consequently, the auxiliary driving mechanism and the coupling means can be made for light duty, since they have to exert on the movable contact only a relatively small force in the opening direction at the beginning of the opening operation. In this respect they differ principally from circuit-breakers provided with external driving mechanisms which have also to produce the very great force to accelerate the movable parts. Such mechanisms must be strong and robust and made for heavy duty. In a circuitbreaker according to the invention the force required to accelerate the movable contact is produced, as in the known gas blast circuit-breakers of this kind, almost exclusively by the pressure of the extinguishing gas exerted on the cylinderpiston-system of said contact.

It will be obvious that the use of mechanical coupling means between the movable contact and the external auxiliary driving mechanism makes the provision of a positive indication of the position of said contact and a positive determination of the point of time, at which the isolating-switch has to be put in action after the opening command has been given to the circuitbreaker, very simple. I

The valve-system can be made, in such a manner, that it is kept in its closed position by the pressure of the extinguishing gas contained in the switching chamber, so that said valvesystem produces at least a part of the holding force. The valvesystem then could be formed as a nonbalanced disc valve. Moreover, the readjusting force may be reduced to zero in the closed condition of the circuit-breaker and therefore be produced, if desired, by the auxiliary driving mechanism.

The valve-system may also be constructed, in such a manner, that it is balanced in its closed condition. In that case force the switching chamber. Thenthe auxiliary driving mechanism can be constructed in a simple manner, say formed as a single acting cylinder-piston-system, which has only to open the valve-system to initiate the opening operation of the circuit-breaker.

If the gas blast circuit-breaker is meant for high voltages and for that reason is provided with at least two pairs of cooperat ing contacts connected in series, it is advantageous to interconnect corresponding movable contacts of said paris of contacts and to connect them to a common external auxiliary.

driving mechanism by mechanical coupling means. The movable contacts are then positively interconnected, so that in all interrupting places the cooperating contacts are separated, simultaneously from one another.

The invention will be elucidated with the aid of the accompanying drawing. In the drawing:

FIG. 1 is a vertical sectional view of a gas blast circuitbreaker according to the invention provided with two pairs of cooperating contacts connected in series;

FIG. 2 is a vertical sectional view of another gas blast circuit-breaker according to the invention provided with one pair of pooperating contacts, which are both movably mounted; an

FIG. 3 is a vertical sectional view of a pair of contacts of still another gas blast circuit-breaker according to the invention, in which the movable contact cooperates with the holder of the fixed contact as a balanced slight-valve-system.

In FIG. 1 is a metal vessel to be connected with earth. Contained in this vesselare three electrically conductive contactholders 2, 3 and 4, which are kept spaced from one another and fromthe vessel 1 by insulators 5, 6 7 and 8. The contactholders 2 and 4 are-connectedwith connecting conductors 9 and 10 for the connection of the circuit-breaker to the electric circuit to be interrupted. The contact-holder 2 carries a fixed contact consisting of a circular series'of contact fingers 11. Cooperating with this fixed contact is an axially movable nozz'le contact 12 which is associated with a piston 14 mounted for reciprocation in a cylinder 13. This nozzle contact and said piston are mounted for sliding over a stationary tube 15 for the discharge'of extinguishing gas. The discharge tube 15 is connected through a passage 16 formed in the contact-holder 3 to discharge tube composed of tubes 17, 18, 19 of insulting material and leading, for instance to a deionization-chamber I (not shown), in which the pressure increases during the opening operation. The cylinder 13 and the discharge tube 15 are attached to the contact-holder 3.

A same pair of cooperating contacts 11, 12 with all accesseries is provided between the contact-holders 3 and 4.

The movable nozzle contacts 12, 12 are each provided with an annular valve-rim 20, 20', which cooperates with an annular seat 21, 21' carried by the contact-holder 2, 3. This circuitbreaker is only meant to interrupt the electric circuit and to extinguish theswitching are, so that it is mounted in-the electric circuit in series with an isolating-switch (not shown). The circuit-breaker is normallyclosed and the valve-rims 20, 20' are then forced against the seats 21, 21 so that the switching chamber 22 containing extinguishing gas under pressure. is separated from the discharge passages 15, 15', 16 16', 17, 18, 19. The switching chamber 22 is supplied with extinguishing gas through the conduit 23.

In accordance with the invention the movable contacts 12, 12' are connected by rods 24, 25 of insulating material and a metal rod 26 with one another, with a piston 28, which is mounted for reciprocation in a cylinder 27 and is exposed to the pressure of the extinguishing gas to produce a readjusting force, and with an auxiliary driving mechanism which consists of a stationary piston 29 and a movable cylinder 30 connected with the rod 26by an arm 31.

in the closed condition of the circuit-breaker the extinguishing gas contained in the switching chamber acts, in the closing direction, onthe piston 28 and on the annular surfaces bounded by the outer diameters of the valve-rims 20, 20' and the outer diameters of the nozzle contacts 12, 1 2" and, in the opening direction, on the annular surfaces bounded by the outer diameter of the pistons 14, 14' and the outer diameters of the nozzle contacts 12, 12'. The force exerted by the extinguishing gas in the closing direction on the movable nozzle contacts 12, 12' isgreater thanthe force exerted in the opening direction on said contacts, so thatsaid contacts are kept in the closed position and the valve-systems 20, 21 and 20', 21 are kept closed by a holding force which is equal to the difference of the forces exerted by the extinguishing gas on the movable contacts and the piston 28'. When the construction and the dimensions of the movable contacts l2, l2 and the cylinder-piston-system 27, 28 are chosen in the right way, the holding force can have, at a given pressure of the extinguishing gas, the smallest allowable value, so that the auxiliary driving mechanism has only to overcome said minimum holding force to initiate the opening operation, that means to open the valve-systems 20, 21 and 20', 21'. if the proportions are selected well the auxiliary driving mechanism 29, 30 and the coupling rods 24, 25 can be made for light duty.

As soon as the valve-rims 20, 20' and the seats 21, 21' have been separated from one another, the pressure of the extinguishing gas exerted in the closing direction on the annular surfaces of the valve parts ceases to exist and larger annular piston surfaces, that meansthe surfaces bounded by the outer diameters of the pistons 14, 14' and the outer diameters of the discharge tubes 15, 15', are exposed to the pressure of the extinguishing gas then acting inthe opening direction, so that the movable contacts 12, 12' are driven by said gas with great speed in the opening direction. In addition the opening moveattached to a cylinder-piston-syste'm 40, 41 for its movement in the opening direction. Inthe same manner the contactholder 37 carries a second movable nozzle contact 42 which is provided for sliding movement on a discharge tube 43 andis attached to a cylinder-piston-system 44, 45 for its movement in the opening direction. The nozzle contact 38 is provided with a circular series of contact fingers 46 which embrace the free end portion of the nozzle contact 42, when the circuitbreaker is in its closed conditionjln addition the nozzle contacts 42 and 38 form together a nonbalanced valve-system and to that end they are provided with a seat 47 and a valve-rim 48 cooperating with said seat. The s'eatand the valve-rim 47, 48 separate, in the closed condition of the circuit-breaker, the switching chamber 49 containing extinguishing gas under pressure from a gas discharge conduit 50 which is in open communication with the dischargetubes 39 and 43. The contacts 38 and 42 are loaded'by readjusting springs 51, 52 which drive the contacts towards each other, as soon as the pressure difference over the pistons 41, 45 has become sufficiently low after the circuit-breaker has reached its opened end position. The nozzle contact 42 is connected by a coupling rod consisting of insulators 53, 54 and metal rods 55, 56 to an external auxiliary driving mechanism which consists of a stationary piston 57 and a movable cylinder 58 connected with the ro 56byanarm59.

In the closed condition of this circuit-breaker, the contacts 38 and 42 are kept engaged with one another by the pressure of the extinguishing gas and the readjusting springs 51, 52 and the valve-rim 48 is forced against the seat 47, so that the extinguishing gas cannot escape from the switching chamber 49. However, as soon as the auxiliary driving mechanism 57, 58 has opened, after having received an opening command, the valve-system constituted by the rim 48 and the seat 49, both contacts 38, 42 are driven with great speed in the opening direction by the extinguishing gas. The switching arc is then extinguished by the extinguishing gas escaping from the switching chamber 49 through the tubes 39 and 43 to the discharge conduit 50.

The circuit-breaker, of which only one pair of cooperating contacts is shown in FIG. 3, is provided with stationary contact holders 60, 61, each of which carries an extinguishing gas discharge tube 62, 63. The contact-holder 60 also carries a fixed contact consisting of a circular series of contact fingers 64 and the contact-holder 61 is provided with an axially movable nozzle contact 65 cooperating with said fixed contact and internal cylindrical surface 68 formed on the movable nozzle contact 65 constituted together with an external cylindrical surface 69 of a ring member 70 carried by the contact-holder '60 and provided with a packing ring 71 a slide-valve-system for separating, in the closed position of the circuit-breaker, the switching chamber 72 containing gas under high pressure from the discharge tubes 62 and 63. The specialty of this circuit-breaker is that the external diameter of the piston 67 is equal to the diameter of the cylindric surface 68, so that the slide-valve-system is balanced, when the circuit-breaker is in its closed position. In this case the readjusting force and the holding force are produced by a cylinder 73 and a piston 75 which ismounted on the coupling rod 74 and loaded by the extinguishing gas in the switching chamber 72. Obviously, a readjusting-holding-spring operating either on said coupling rod 74 or directly on the nozzle contact 65 may be used for that purpose. The auxiliary driving mechanism to open the operation of the circuit-breaker is not shown. I

it is observed, that for the auxiliary driving mechanism for gas blast circuit-breakers of the present kind a mechanical, an electromagnetical, a pneumatic or an hydraulic driving mechanism or a combination of such mechanisms may be used.

It will be apparent that the readjusting force which must drive the movable switching contacts, back to their closed position directly after the switching arcs have been extinguished, has to overcome the pressure difference acting on said switching contacts in the opening direction at the right moment. This pressure ditference'which is maximal at the point of time, at which the valve-systems are just opened, and which drives the movable contacts, so to say with a blow, towards their opened end positions then decreases, since the pressure in the discharge passages and the pressure exerted in the closing direction on said contacts increase due to the counter-pressure of the escaping extinguishing gas. The condi- 'tions may then be so chosen as'to make a small readjusting force sufficient to drive the circuit-breaker back to its closed position. Should the discharge passages, as in many cases, be connected to so-called expansion chambers, in which the escaped extinguishing gas is collected and from which said gas is discharged later on, that means after it has been deioninzed, the pressure in the discharge passages is at first quickly increased, so that the readjusting force'may be made very small. As already said, this readjusting force may be permanently present and it may be produced by a readjusting piston loaded by the pressure prevailing in the switching chamber or be furnished by readjusting springs. However, it is also possible to use the auxiliary driving mechanism to produce the readjusting force. Said mechanism must then be operated in the opposite direction. In thelatter case the readjusting force is reduced to zero during the open operation, so that it does not counteract said opening operation.

It is a great additional advantage of the gas blast circuitbreaker constructed in accordance with the invention that both the control of the isolator-switch connected in series with the circuit-breaker and used for the permanent interruption and the closing of the electric circuit and the indication of the position of the movable switching contacts can be easily derived from the position of the coupling rod. Finally the coupling rod acts also as a mechanical connection between the movable contacts of pairs'of cooperating contacts connected in series, having that during the opening operation therein, contacts are moved at the same time and in the same manner.

We claim:

1. A gas blast circuit-breaker comprising a container having inner and outer compartments formed therein, the outer compartment being filled with extinguishing gas under pressure, at least one pair of cooperating contacts contained in the inner compartment, at least one of said contacts being a movable contact and at least one of said contacts having a central passage for the discharge of extinguishing gas during the opening operation of the circuit-breaker, said discharge passage being in open communication with said innercompartment, a piston attached to said movable contact andadapted to drive said movable contact in the opening direction, said piston having one side permanently exposed to the pressure of the gas contained in the outer compartment for urg'ing said movable contact in the opening direction, a valve surrounding said inner compartment and constituted by two cooperating valve members, of which one is fixed to one contact and the other one is fixed to the other contact of said pair of cooperating contacts, said valve being closed and separating the inner compartment and the discharge passage from the outer compartment when the circuit-breaker is in its closed condition, and said valve being opened directly after the opening operation of the circuit-breaker has been started so as to allow extinguishing gas from the outer compartment to escape through the inner compartment and the discharge passage, said valve being held in its closed condition by a force produced by the extinguishing gas contained in the outer compartment and counteracting the force exerted on said piston by said gas, means to produce a holding force to keep the circuit-breaker and the valve in their closed conditions, and an external auxiliary driving mechanism adapted to start the opening operation of the circuit-breaker and to open said valve against said holding force.

2. A gas blast circuit-breaker according to claim 1, in which the force exerted on the valve by theextinguishing gas contained in the outer compartment, when the circuit-breaker is in its closed condition, is greater than the force exerted on the piston of the movable contact by said gas, the difference between said forces producing at least a part of the holding force.

3. A gas blast circuit-breaker according to claim 1, in which the force exertedon the valve by the extinguishing gas contained in the outer compartment, when the circuit-breaker is in its closed condition, is equal to the force exerted on the piston of the movable contact by said gas and in which a second piston is provided, said second piston being also coupled with the movable contact and having its side for the closing action on said contact permanently exposed to the pressure of the gas contained in the outer compartment, the holding force being produced by said second piston.

4. A gas blast circuit-breaker according to claim 3, in which said second piston is fixed on a coupling rod connecting the movable contact to the extemal-auxiliary driving mechanism.

5. A gas blast circuit-breaker as claimed in claim 1, in which at least two pairs of cooperating contacts are electrically connected in series, corresponding movable contacts of said pairs of contacts being connected by mechanical coupling means to one another and to a common external auxiliary driving mechanism.

6. A circuit-breaker comprising in combination! an enclosure defining a first chamber; means for supplying pressurized insulating gas to said chamber normally to maintain said chamber at a predetermined pressure;

contact means within said chamber for defining, under one condition, an inner chamber isolated from said first chamber;

means for venting said inner chamber whereby a pressure differential prevails between said chambers under said one condition of said contact means; said contact means including a sleevelike axially movable contact and a second contact engaged by one end of said sleevelike contact when said contact means is in said one condition, said sleevelike contact having a radially enlarged valve portion adjacent said one end thereof and a radially enlarged piston portion adjacent the other end thereof, said valve portion being in sealing contact with said second contact when said contact means is in said one condition whereby to maintain said pressure differential, and said valve portion and said piston portion being dimensional so that the forces exerted thereon due to said-pressure differential and which forces act in tipposite directions, are at least substantially mutually cancelled;

additional means for urging said contact means to said one condition thereof whereby a small net force acts to maintain such one condition; and

means including an external actuator and an actuating rod connecting said actuator to said sleevelike contact for overcoming small net force to separate said contacts and open said inner chamber to 'said first chamber so that the insulating gas rushes past said contacts and axially through said sleeve-like contact.

7. The circuit-breaker according to claim 6, wherein said additional means comprises a piston on said actuating rod exposed on one side thereof to said first chamber.

8. The circuit-breaker according to claim 6, wherein said additional means comprises springs acting on said sleevelike contact.

9. A circuit-breaker comprising, in combination:

an enclosure defining a chamber;

means for supplying pressurized extinguishing gas to said chamber normally to maintain said chamber at a predetermined pressure; I

a pair of electrically conductive members disposed in spaced relation to each other within said chamber, at least one of said members being fixed and including a tubular extension projecting therefrom toward the other member and a surrounding wall portion defining an annular cylinder with the outer surface of said tubular extens|on;

a first electrical contact supported by said other member in spaced relation beyond the free end of said tubular extensron;

a second sleevelike electrical'contact movable axially of said tubular extension between a closed condition in which one end thereof engages said first contact and an open condition in which said one end thereof is separated from said one contact, said one end of the second contact having a radially projecting shoulder thereon and said 7 other member having an annular surface portion sealingly engaged by said shoulder when said contacts are in closed condition, and the other end of said second contact having radially projecting piston portion engaging said wall portion of said annular cylinder whereby to define an isolated from the first mentioned inner chamber chamber;

means for venting said inner chamber whereby the pressure difference due to said pressurized insulating gas acting against said shoulder tends to maintain said contacts in closed condition while the pressure difference acting against said piston portion tends to separate said contacts;

additional means for urging said contacts toward closed condition in any relative position of such contacts, said additional means and the dimensions of said shoulder and said piston portion being such that, when said contacts are in closed condition, a small net force maintains said contacts in closed condition when said chamber is subjected to said predetermined pressure; and

means including an external actuator and an actuating rod connecting said actuator to said one contact for overcoming said small net force to separate said contacts. 

1. A gas blast circuit-breaker comprising a container having inner and outer compartments formed therein, the outer compartment being filled with extinguishing gas under pressure, at least one pair of cooperating contacts contained in the inner compartment, at least one of said contacts being a movable contact and at least one of said contacts having a central passage for the discharge of extinguishing gas during the opening operation of the circuit-breaker, said discharge passage being in open communication with said inner compartment, a piston attached to said movable contact and adapted to drive said movable contact in the opening direction, said piston having one side permanently exposed to the pressure of the gas contained in the outer compartment for urging said movable contact in the opening direction, a valve surrounding said inner compartment and constituted by two cooperating valve members, of which one is fixed to one contact and the other one is fixed to the other contact of said pair of cooperating contacts, said valve being closed and separating the inner compartment and the discharge passage from the outer compartment when the circuit-breaker is in its closed condition, and said valve being opened directly after the opening operation of the circuit-breaker has been started so as to allow extinguishing gas from the outer compartment to escape through the inner compartment and the discharge passage, said valve being held in its closed condition by a force produced by the extinguishing gas contained in the outer compartment and counteracting the force exerted on said piston by said gas, means to produce a holding force to keep the circuit-breaker and the valve in their closed conditions, and an external auxiliary driving mechanism adapted to start the opening operation of the circuit-breaker and to open said valve against said holding force.
 2. A gas blast circuit-breaker according to claim 1, in which the force exerted on the valve by the extinguishing gas contained in the outer compartment, when the circuit-breaker is in its closed condition, is greater Than the force exerted on the piston of the movable contact by said gas, the difference between said forces producing at least a part of the holding force.
 3. A gas blast circuit-breaker according to claim 1, in which the force exerted on the valve by the extinguishing gas contained in the outer compartment, when the circuit-breaker is in its closed condition, is equal to the force exerted on the piston of the movable contact by said gas and in which a second piston is provided, said second piston being also coupled with the movable contact and having its side for the closing action on said contact permanently exposed to the pressure of the gas contained in the outer compartment, the holding force being produced by said second piston.
 4. A gas blast circuit-breaker according to claim 3, in which said second piston is fixed on a coupling rod connecting the movable contact to the external auxiliary driving mechanism.
 5. A gas blast circuit-breaker as claimed in claim 1, in which at least two pairs of cooperating contacts are electrically connected in series, corresponding movable contacts of said pairs of contacts being connected by mechanical coupling means to one another and to a common external auxiliary driving mechanism.
 6. A circuit-breaker comprising in combination: an enclosure defining a first chamber; means for supplying pressurized insulating gas to said chamber normally to maintain said chamber at a predetermined pressure; contact means within said chamber for defining, under one condition, an inner chamber isolated from said first chamber; means for venting said inner chamber whereby a pressure differential prevails between said chambers under said one condition of said contact means; said contact means including a sleevelike axially movable contact and a second contact engaged by one end of said sleevelike contact when said contact means is in said one condition, said sleevelike contact having a radially enlarged valve portion adjacent said one end thereof and a radially enlarged piston portion adjacent the other end thereof, said valve portion being in sealing contact with said second contact when said contact means is in said one condition whereby to maintain said pressure differential, and said valve portion and said piston portion being dimensional so that the forces exerted thereon due to said pressure differential and which forces act in opposite directions, are at least substantially mutually cancelled; additional means for urging said contact means to said one condition thereof whereby a small net force acts to maintain such one condition; and means including an external actuator and an actuating rod connecting said actuator to said sleevelike contact for overcoming small net force to separate said contacts and open said inner chamber to said first chamber so that the insulating gas rushes past said contacts and axially through said sleeve-like contact.
 7. The circuit-breaker according to claim 6, wherein said additional means comprises a piston on said actuating rod exposed on one side thereof to said first chamber.
 8. The circuit-breaker according to claim 6, wherein said additional means comprises springs acting on said sleevelike contact.
 9. A circuit-breaker comprising, in combination: an enclosure defining a chamber; means for supplying pressurized extinguishing gas to said chamber normally to maintain said chamber at a predetermined pressure; a pair of electrically conductive members disposed in spaced relation to each other within said chamber, at least one of said members being fixed and including a tubular extension projecting therefrom toward the other member and a surrounding wall portion defining an annular cylinder with the outer surface of said tubular extension; a first electrical contact supported by said other member in spaced relation beyond the free end of said tubular extension; a second sleevelike electrical contact movable axially of said tubular extensIon between a closed condition in which one end thereof engages said first contact and an open condition in which said one end thereof is separated from said one contact, said one end of the second contact having a radially projecting shoulder thereon and said other member having an annular surface portion sealingly engaged by said shoulder when said contacts are in closed condition, and the other end of said second contact having radially projecting piston portion engaging said wall portion of said annular cylinder whereby to define an inner chamber isolated from the first mentioned chamber; means for venting said inner chamber whereby the pressure difference due to said pressurized insulating gas acting against said shoulder tends to maintain said contacts in closed condition while the pressure difference acting against said piston portion tends to separate said contacts; additional means for urging said contacts toward closed condition in any relative position of such contacts, said additional means and the dimensions of said shoulder and said piston portion being such that, when said contacts are in closed condition, a small net force maintains said contacts in closed condition when said chamber is subjected to said predetermined pressure; and means including an external actuator and an actuating rod connecting said actuator to said one contact for overcoming said small net force to separate said contacts. 